This invention generally relates to an insert for a ball nose end mill, and is specifically concerned with such an insert having sinusoidal undulations along its cutting edge for reducing cutting forces and vibration and facilitating the breaking and removal of chips.
Inserts for ball nose end mills are well known in the prior art. Such inserts typically comprise an integral body formed from a hard, wear resistant material having at least one arcuate cutting surface that may be quadrant-shaped. The end mill itself includes an elongated cylindrical body having a shank portion for attachment to a turning tool, and a hemispherically-shaped end having a quadrant-shaped seat for receiving and mounting the insert. The portion of the cutting edge nearest the tip of the hemispherically-shaped end crosses over the axis of rotation of the end mill a short distance to insure that the cutting edge of the insert engages the workpiece along the axis of rotation of the end mill body, thus allowing the end mill to perform a plunge operation in a workpiece.
Such ball nose end mills have proven to be highly versatile machine tools that are capable of performing plunge-type cutting much like a drill, or face-type milling like a conventional milling head, or even ramp-type machine operations that combine the motions of both plunge and face-type cutting. However, the applicants have noted a number of shortcomings in the performance of the insert used with such end mills where performance could be substantially improved. For example, the applicants have noted that the region of the cutting edge that intersects the axis of rotation is subjected to large amounts of heat-generating shear forces since the rotational speed of the edge is zero at the axis, and very slow in the portion of the edge adjacent to the axis. Some insert designers have attempted to solve this problem by shaping the cutting edge so that it falls short of traversing the axis of rotation. Unfortunately, such a design necessarily creates a small protrusion of uncut workpiece material at the point where the axis of rotation of the end mill body intersects the workpiece. While the unwanted protrusion of uncut material is periodically broken off due to the forces applied to it by the cutting operation, a small rough spot along the axis of rotation can be created on the surface of the cut.
Still other shortcomings include the relatively higher cutting forces and vibrations associated with the use of a prior art ball nose end mill versus the use of a more conventional (but unfortunately less versatile) milling cutter. The applicants have observed that one of the causes of such higher cutting forces and vibration is that all points of the cutting edge of the inserts used in such end mills orthogonally engage the workpiece at all times during the cutting operation. While it is possible to reduce the cutting forces and vibrations by mounting the insert at an angle with respect to the axis of rotation of the cutter body (thereby imparting an axial rake angle to the insert), such a technique requires the provision of relatively deep insert seats, which in turn weakens the body of the cutter. Additionally, such a tilted mounting of the insert can produce an unwanted concavity in the sidewalls of the cut made by the end mill, thus creating a distortion in the shape of the cut when a true hemispherical profile is desired.
Finally, the applicants have noted that some inserts for such ball nose cutters do not effectively embrittle the metal chips that result from certain cutting operations. Hence, if such a cutter is used to implement a fine-cut plunge operation in a highly ductile material, the insert used in the cutter may not effectively embrittle the resulting foil like chips, which can interfere with their expulsion from the chip-expelling flute of the cutter body and thus interfere with the cutting operation.
Clearly there is a need for an insert for use in a ball nose end mill that is capable of producing rounded cuts in a workpiece without the generation of large stresses and frictional heat where the cutting edge intersects the center line of the cutter. Ideally, such a cutter should be able to cut a workpiece with lower cutting forces and with less vibration than the inserts of the prior art without the need for tilting the insert at a substantial axial rake angle, thereby reducing power requirements while increasing tool longevity. Finally, it would be desirable if such an insert were capable of imparting substantial embrittling forces to the chips resulting from a cutting operation so that even very thin chips formed from highly ductile metals will curl and break into small pieces during a cutting operation.